Construct diffs with attached changesets, even if empty

Summary: See discussion in IRC. Not 100% sure what's going on here because of email ghost theives, but conceivably a commit with no changes will end up with `null` changesets instead of `array()` changesets, which throws. Such diffs are certianly possible (`git commit --allow-empty`) even if they aren't the issue in this specific case. See T3416. Initialize changesets to `array()` to avoid throwing.

Test Plan:
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iiam

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6339

src/applications/differential/storage/DifferentialDiff.php

@@ -115,6 +115,10 @@ final class DifferentialDiff extends DifferentialDAO {
     assert_instances_of($changes, 'ArcanistDiffChange');
     $diff = new DifferentialDiff();
 
+    // There may not be any changes; initialize the changesets list so that
+    // we don't throw later when accessing it.
+    $diff->attachChangesets(array());
+
     $lines = 0;
     foreach ($changes as $change) {
       if ($change->getType() == ArcanistDiffChangeType::TYPE_MESSAGE) {